A transmission is any mechanical linkage that converts an input torque to an output torque. It usually involves a series of gears that have differing diameters, allowing a first gear at a first rotation rate to link to a second gear rotating at a second rate. The most common application for transmissions is in a vehicle. For example, a car may have an automatic transmission or a manual transmission. A bicycle has a simple transmission that links the pedals to the hub of the rear wheel.
Transmissions allow an input force to be converted into a more useful and appropriate output. However, by using gears and linkages, a typical transmission may only have four or five ratios available. For example, a four speed automatic transmission in a car has only four sets of output gears to couple to the engine's input. A ten speed bike has only ten ratios of input to output. A need exists for a transmission that is not limited by the number of gears. Yet, to place a larger number of gears into a transmission increases its costs and weight and space requirements.
A continuously variable transmission (CVT) or continuously variable planetary (CVP) is a transmission that eliminates the need for a specified number of gears. Instead it allows an almost limitless number of input to output ratios. This is a benefit because it allows an output to be achieved, i.e. the speed of a vehicle, at an optimal input, i.e. the rpm of the engine. For example, an engine might be most efficient at 1800 rpm. In other words, the peak torque output for the engine might be achieved at this engine rpm, or perhaps the highest fuel economy. Consequently, it may be desirable to run at a specified RPM for an economy mode or a power mode. Yet, in third gear, the car might be going faster at 1800 rpm than the driver desires. A continuously variable transmission would allow an intermediate ratio to be achieved that allowed the optimal input to achieve the desired output.
CVTs have a variator for continuously variable adjustment of the ratio. A customary structure is a belt drive variator having two pairs of beveled pulleys and rotating a torque-transmitter element therein, such as a pushing linked band or a chain. The beveled pulleys are loaded with pressure from the transmission oil pump in order, on one hand, to actuate the ratio adjustment and, on the other, to ensure a contact pressure needed for transmission of the torque upon the belt drive element. Another usual structure is a swash plate variator in semi-toroidal or fully toroidal design.
Examples of CVTs are exemplified by U.S. Pat. Nos. 6,419,608 and 7,011,600 assigned to Fallbrook Technologies of San Diego, Calif. In each of those references the axial movement of a rod or an axial force as indicated by numeral 11 of each reference is used to vary the input to output ratio of such transmissions. While a continuously variable transmission is artful on paper, the realities of making one work smoothly requires significant know how. Consequently, a need exists for a system that permits axial shifting of the rod 11.
While CVTs have primarily been applied to more conventional vehicles such as motor scooters and bicycles, wheelchairs represent another class of transport that has been inadequately equipped over the years.